1. Field of the Invention
The present invention relates generally to the construction of a sensor incorporating a heater, and more particularly to a sensor which has thermally insulating means for preventing dissipation or transfer of heat generated by heating means incorporated therein, and gas-inlet means for introducing a measurement gas to a detecting portion which has a measuring electrode.
2. Discussion of the Prior Art
An oxygen sensor using an electrochemical element or cell having a tubular solid electrolyte body is known, for example, in the field of controlling an amount of oxygen contained in an exhaust gas emitted from an internal combustion engine. The tubular solid electrolyte body is closed at its one end, and has a pair of electrodes on substantially the entire areas of its opposite inner and outer surfaces. In this type of oxygen sensor, a protective covering member is provided so as to surround the detecting end portion of the electrochemical cell, to protect the sensor from contamination by foreign substances contained in the exhaust gas, or from mechanical damages during installation of the sensor. Such a protective covering member has apertures or openings formed over its entire area, so that the entire area of the measuring electrode formed on the outer surface of the tubular solid electrolyte body may be evenly exposed to the measurement gas, and so that the electrochemical cell may be effectively heated by the hot exhaust gas to a suitable operating temperature.
However, such an oxygen sensor suffers from a drawback that immediately after a cold engine is started, the temperature of the exhaust gas is not high enough to heat the electrochemical cell to a sufficiently high operating temperature, and therefore the electrochemical cell cannot operate in a reliable manner until the exhaust gas has become sufficiently hot.
To solve the above drawback, it is known to provide an oxygen sensor with suitable heating means such as an electric heater, which is disposed in direct or indirect contact with the electrochemical cell of the sensor.
Unlike the conventional oxygen sensor which utilizes the heat of the hot exhaust gas to heat the electrochemical cell, the oxygen sensor incorporating such heating means to heat the solid electrolyte body employs a protective covering means or device which has suitable thermally insulating means for preventing the solid electrolyte body from being cooled by the cool exhaust gas immediately after the start of a cold engine. For example, the thermally insulating means may be provided by reducing the size of the gas-inlet openings formed in the covering means, or by using two covering members which form a double-walled protective covering device.
However, the conventional protective covering device used for the conventional oxygen sensor incorporating heating means is not completely satisfactory in terms of its effect on an operating response of the sensor and its effect to thermally insulate the solid electrolyte body. Namely, the operating response is deteriorated if the total area of opening in the gas-inlet portion of the covering device is excessively reduced for preventing dissipation of the heat generated by the heating means. Conversely, the effect of the thermally insulating portion of the covering device is sacrificed if the total area of opening is excessively increased for assuring a comparatively high operating response of the sensor.